Make node discovery configurable

[rubo]

Currently, there's no way to configure the ENR tree lookup. It's hardcoded. There's should be an option like the one of Geth.

Requested on Discord:

Is there any documentation available about how to use DNS discovery (EIP-1459) with Nethermind? So after we have a signed txt record and compiled a list of nodes, how and where should we configure the domain name? I mean, there should be a place in our configuration that points to the DNS name so that it can be consumed (fetch the txt record, validate the signature, read the list, etc) by the node.